there is no ultimate rod unless they just made one for platinum, but there is a super rod which is the best rod in the game. to get that you need to get the national dex then go to the fight area and go to the route the doesn't have water. before you go into the thing that brings you into the actual route there is a fisherman and he gives you it.
Old Rod.
In Pokemon Platinum the Good Rod is to the right of Hearthome City. The fisherman on Route 209 will give you the good rod.
a good rod or super rod
You get the old rod to the left of jubilife city.
On Pokemon Platinum, Carvanha can be caught in the Great Marsh.PLEASE NOTE: REQUIRES SUPER ROD.
fish with the super rod at the pokemon league
route 218 (good rod)
You can receive a Super Rod from a fisherman in the Fight Area on Pokemon Platinum, Diamond and Pearl.
West of jubilife city.
left of pastoria city
fisherman on route 209
left of pastoria city